This invention relates to an optical head for recording, reproduction and erasing used in an optical disk memory system and in particular to an optical head using a thin film optical wave guide.
As the post-industrialization progresses, the amount of information treated in the society is surely increasing. For this reason the request to increase the memory capacity of the information recording device storing this information becomes progressively stronger. As a system for recording or reproducing various sorts of information those utilizing a magnetic method or an optical method are used in practice. However, among them, an optical disk system utilizing an optical disk as a memory medium permits storage of information with a density, which is more than 100 times as high as that of a prior art system utilizing a magnetic medium, and on the other hand it has an excellent feature that high quality reproduced signals can be obtained without any contact. Video disk, digital audio disk, etc. are known as memory systems utilizing such an optical recording medium.
FIG. 1 illustrates the basic construction of an optical head used in these systems. In the figure a laser light beam emitted by a semiconductor laser device 1 is transformed into a parallel light beam by a collimator lens 2 and goes straight on through a beam splitter 3. It passes further through a 1/4 wave plate 4 and an objective lens 5 and is focused on an optical disk 6 so as to form a light spot 7. This light spot 7 is reflected by the optical disk 6 and passes again through the objective lens 5 and the 1/4 wave plate 4. Then it passes through cylindrical lenses 8 and 9 after having been reflected to the right angle by the beam splitter 3, which is a half mirror, and is focused on an optical sensor 10. This optical sensor 10 detects the focusing and the tracking information of the objective lens 5 and the presence or absence of the record on the disk.
In such an optical system it was necessary to regulate the position of each of the optical parts so that the light spot is focused on the optical disk 6 and the optical sensor 10. There was a problem that the regulation necessitated a long time. Further there was another problem that the reduction of the size of the optical head was limited.
Since the reduction of the size of the optical head contributes considerably to the reduction of the size of the whole optical disk system and to the increase of the reliability, research and development therefor are at present widely promoted. As a proposal for the reduction of the size and the weight of an optical head, that disclosed by JP-A-60-202553 is known. According to this proposal, indicated in FIG. 2, an optical wave guide 12 is formed by diffusing titanium into a surface portion several microns thick of a substrate 11 made of lithium niobate (LiNbO.sub.3) and a laser light beam emitted by a semiconductor laser device 13 is projected to one end surface of this optical wave guide 12. Then a light spot 16 is formed by focusing it on the other end surface of the optical wave guide 12 by means of refractive index distribution type lenses 14 and 15.
This kind of techniques is summarized in the following lierature; T. SUHARA et al., "Integrated Optics Components and Devices Using Periodic Structures" IEEE J. of Quantum Electronics pp. 845-867, 1986.
Since, in the prior art techniques, no attention was paid to the light spot actuate techniques, i.e. autofocusing and autotracking, in order to realize these functions, the practice was to locate the whole optical head on a coil actuator, etc. and to move it forward and backward toward left and right. For this reason there was a problem that the construction was complicated and that the reliability of the autofocusing and the autotracking was low.